In recent years, in non-aqueous electrolyte secondary batteries, lithium ion secondary batteries having a high voltage and a high energy density have been actively studied. In general, in lithium ion secondary batteries, a lithium-containing transition metal oxide such as LiCoO2 is used as a positive electrode active material, a carbon material is used as a negative electrode active material, and a porous film made of polyethylene or polypropylene is used as a separator. A non-aqueous electrolyte generally includes a non-aqueous solvent and a solute dissolved therein. For the non-aqueous solvent, a cyclic carbonic acid ester, a chain carbonic acid ester, a cyclic carboxylic acid ester, and the like are used, and for the solute, lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), and the like are used.
Various attempts have been made to improve the positive electrode active material, the negative electrode active material, the separator, and the non-aqueous electrolyte, for the purpose of improving the battery characteristics. With respect to the separator, for example, the following improvements have been made.
Patent Document 1 suggests, in order to further improve the safety of batteries in the event of short circuit or in abnormal use, using a separator formed by laminating a porous film made of fluorocarbon resin such as polytetrafluoroethylene (PTFE), and a polyethylene or polypropylene film. In Patent Document 1, since the separator includes a fluorocarbon resin film having a high melting point, the melting of the separator in the event of abnormal heat generation can be prevented. As such, the safety of batteries can be further improved.
Patent Document 2 suggests, in order to further improve the safety of batteries including a metallic lithium as the negative electrode active material, using a separator comprising two layers having different pore sizes. The layer having a smaller pore size prevents the dendritic growth of the metallic lithium, and as a result, an internal short circuit that may occur during charge and discharge and an ignition that may occur in association with the internal short circuit can be prevented. Specifically, Patent Document 2 discloses a separator formed by laminating a polytetrafluoroethylene film and a polypropylene film having a small pore size.
On the other hand, with respect to the non-aqueous electrolyte, for example, the following improvements have been made.
In Patent Document 3, by adding fluorinated benzenes into the non-aqueous electrolyte, the cycle performance is improved.
Patent Document 4 suggests using monofluoroethylene carbonate as the non-aqueous solvent of the non-aqueous electrolyte. In Patent Document 4, since a stable coating film derived from monofluoroethylene carbonate is formed on the negative electrode, the improvement in cycle performance is achieved.
Patent Document 1: Japanese Laid-Open Patent Publication No. Hei 5-205721
Patent Document 2: Japanese Laid-Open Patent Publication No. Hei 5-258741
Patent Document 3: Japanese Laid-Open Patent Publication No. 2005-340026
Patent Document 4: Japanese Laid-Open Patent Publication No. 2004-063432